A thermal printhead includes an insulating substrate and a heating resistor formed on the substrate. In printing, current is applied to the heating resistor, whereby images or letters are formed on a recording medium such as thermal paper (see Patent Document 1 below).
FIG. 5 is a partial sectional view showing an example of conventional thermal printhead. The thermal printhead X shown in the figure includes a substrate 91 and a partial glaze 92 elongated in the primary scanning direction. A plurality of electrodes 93 extending in the secondary scanning direction are provided on the substrate 91. A heating resistor 94 extending across the electrodes 93 (i.e., in the primary scanning direction) is provided on the partial glaze 92. A protective film 95 for protecting the heating resistor 94 and other parts is provided on the substrate 91. In printing, thermal paper is transferred in the secondary scanning direction while being pressed against the protective film 95.
Sticking (the phenomenon in which a recording medium sticks to a printhead) is a conventionally known problem which often occurs in a thermal printhead. To prevent sticking in the above-described printhead X, it is necessary to form the protective film 95 properly. The provision of the proper protective film 95 is also necessary for increasing the printing speed.
For instance, to make the protective film 95 using a material having a high thermal conductivity may be considered to be an effective way to increase the printing speed. With such a protective film, the thermal paper is heated and cooled quickly, so that the printing speed is expected to increase.
However, the rapid temperature change of the thermal paper is not suitable for preventing the sticking. Specifically, a resin material for fixing a heat-sensitive material is applied to the thermal paper. When the resin material is suddenly cooled after melted due to the temperature rise, the thermal paper tends to adhere to the protective film 95.
Patent Document 1: JP-A-2002-2005